In the operation of existing devices of the character known as powdered flame spray guns, a very fine particulate thermoplastic material is heated to its melting point, such as by a propane flame. The resultant melted material is then propelled against the surface of article to be coated by means of a stream of propelling air, whereupon the molten material hardens to form the desired surface coating.
However, problems have been associated with such techniques in achieving the proper temperature and manner of mixture of the various spray ingredients, and in the manner of projecting the melted plastic against the article surface.
In a spray gun of the type disclosed in U.S. Pat. No. 4,632,309, an open-atmosphere powdered flame spray gun and a method of spray application are disclosed, in which a powderized thermoplastic material, combustion air, and a combustion gas are delivered through a plurality of passageways extending through the spray gun body into an open mixing and combustion chamber defined by a cylindrical hood about the spray gun body. The resultant mixture is ignited and the thermoplastic material is melted in the flame combustion area entrained in a stream of pressurized air which then deliverers the melted material to the desired surface to be coated. The disclosed method and apparatus were commercially successful, however, certain limitations and disadvantages were discovered to be inherent in the prior design and method.
One limitation discovered was that projecting the stream of combustible gas into the combustion chamber at an oblique angle toward the axis of the combustion chamber and toward the central stream of propelling air and entrained particulate material actually caused a "pinching" of the stream of particulate material and limited the quantity of thermoplastic material that could be melted and delivered for spray coating. In addition, the angular delivery of the combustible gas in the combustion chamber was found to limit the size of the flame "tunnel" emanating from the combustion chamber, and therefore was a self-limiting factor in the total quantity of particulate thermoplastic material that could be melted for spray application. Further, if increased flow rates of particulate material were desired to be delivered by the spray gun, an improved hopper and eductor feed means were necessary to entrain and mix the desired quantity of particulate material in the stream of propelling air.
Accordingly, an improved method and apparatus for spraying a greatly increased quantity of molten particles is disclosed in which the previously described problems associated with previous methods and apparatus are overcome by the present invention and a novel method and apparatus for applying powdered flame sprays is disclosed.